Update installation instructions

If a user installs scala-mode2 via elpa we don't want to encourage them to 'require scala-mode2'
as most users will stick this in their .emacs which won't work since elpa packages are loaded *after*
the emacs init file.
Add .sbt to config
Merge pull request #49 from zbskii/master

Update install instructions to give correct instructions on using ELPA